Low-Temperature Perovskite Scintillators and Devices With Low-Temperature Perovskite Scintillators

ABSTRACT

Disclosed embodiments include perovskite scintillators configured to be operated at a low temperature, detectors with perovskite scintillators configured to be operated at a low temperature, scanners with perovskite scintillators configured to be operated at a low temperature, methods of cooling a perovskite scintillator to a low temperature, and methods of configuring a perovskite scintillator to be operated at a low temperature.

If an Application Data Sheet (ADS) has been filed on the filing date ofthis application, it is incorporated by reference herein. Anyapplications claimed on the ADS for priority under 35 U.S.C. §§ 119,120, 121, or 365(c), and any and all parent, grandparent,great-grandparent, etc. applications of such applications, are alsoincorporated by reference, including any priority claims made in thoseapplications and any material incorporated by reference, to the extentsuch subject matter is not inconsistent herewith.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the earliest availableeffective filing date(s) from the following listed application(s) (the“Priority Applications”), if any, listed below (e.g., claims earliestavailable priority dates for other than provisional patent applicationsor claims benefits under 35 USC § 119(e) for provisional patentapplications, for any and all parent, grandparent, great-grandparent,etc. applications of the Priority Application(s)).

PRIORITY APPLICATIONS

The present application claims benefit of priority of U.S. ProvisionalPatent Application No. 62/831,992, entitled BRIGHT AND FASTSCINTILLATION OF ORGANOLEAD PEROVSKITE MAPbBr3 AT LOW TEMPERATURES,naming Michael Saliba as inventor, filed Apr. 10, 2019, which was filedwithin the twelve months preceding the filing date of the presentapplication or is an application of which a currently co-pendingpriority application is entitled to the benefit of the filing date.

If the listings of applications provided above are inconsistent with thelistings provided via an ADS, it is the intent of the Applicant to claimpriority to each application that appears in the DomesticBenefit/National Stage Information section of the ADS and to eachapplication that appears in the Priority Applications section of thisapplication.

All subject matter of the Priority Applications and of any and allapplications related to the Priority Applications by priority claims(directly or indirectly), including any priority claims made and subjectmatter incorporated by reference therein as of the filing date of theinstant application, is incorporated herein by reference to the extentsuch subject matter is not inconsistent herewith.

BACKGROUND

Scintillators detect ionising radiation by converting energy depositedin them to a proportional number of photons. Scintillators areomnipresent in large-scale technical and commercial applications aroundus. For example, they are found in many branches of physics, securityscanners, or medical applications such as nuclear imaging (liketomography) and they typically operate at room temperature.

An ideal scintillator emits a maximum number of scintillation photonsper energy deposited, has a high absorption coefficient for gammaquanta, and exhibits a narrow timing profile for its scintillationphotons. Brighter and faster scintillators facilitate better timingresolution—which is important for measuring the time of the initialparticle or radiation interaction with high precision

At present, a major limitation of modern scintillators is timingresolution. State-of-the-art resolution in coincidence timing has justbroken the 100 ps barrier—with the lowest value of 73±2 ps for LSO-Cescintillators and 80±4 ps for LGSO-Ce scintillators. A main limitationtowards a goal of 10 ps resolution is low light yield and long decaytime of scintillators. Currently, the best scintillator in this regardis deemed to be LaBr3-Ce—which exhibits 16 ns decay time and 70000ph/MeV. To achieve a timing resolution below 10 ps, a scintillator witha light yield of at least 140000 ph/MeV and a decay time of 1 ns, orshorter, is entailed.

Perovskites may have applicability as a scintillator with a light yieldof at least 140000 ph/MeV. However, currently-known perovskitescintillators do not have a decay time of 1 ns or shorter. Hybridmetal-halide perovskites such as CsPbX3 (X=Cl, Br or I) have exhibitedknown semiconducting behavior. In addition, tolerance of optical andelectronic characteristics to structural defects made solid-statephotovoltaics based on organic-inorganic trihalide perovskites(OTP)—materials with the general formula MAPbX3 where MA=methylammonium,and X=Cl, Br and I—attractive for various optoelectronic applications.Specifically, high photoluminescence quantum yield of OTPs enabledbright light-emitting devices and lasers, whereas high currentconversion efficiency upon light exposure underpinned their applicationas photodetectors. Photovoltaic OTPs have also stimulated solar cellresearch. However, due to the Shockley-Queisser limit, photovoltaicperovskites are typically tuned to a very narrow band gap range, therebyexcluding a majority of high-quality perovskites (especially those withlarger band gaps).

Higher-mass elements with a correspondingly high atomic number (Z) usedin OTPs, i.e. Pb, Br and I atoms, make OTPs inherently suitable forapplications in which good X-ray absorption capability is required.Furthermore, OTPs exhibit a high mobility of charge carriers, whichmakes them optimal for radiation detection through direct conversion ofX-ray photons into current. Soft X-rays (<10 keV) have been detectedusing the photoelectric effect in polycrystalline MAPbI₃ films.Improving the detection probability for hard X-rays (>100 keV) calledfor a decrease in absorption length. This prompted the development ofX-ray detectors based on OTP single crystals or thick films. The energyspectra measured with OTP detectors demonstrated energy resolutions of35% for 59.6 keV of ²⁴¹Am and 6.5% for 662 keV of ¹³⁷Cs.

Thus, OTPs may be viable candidates to detect ionising radiation. Itwill be appreciated that it may be desirable to avoid limitations thatarise from extracting charged particles. This inherent feature ofphotodetectors with direct photon-to-current conversion imposes twobasic constraints. First, it eventually limits the thickness of theabsorber and, hence, conversion efficiency for high-energy photons.Second, the transit time of charge carriers in the material dictates therelatively slow (˜100 μs) response time of OTP photodetectors. It willbe appreciated that an advantage of a scintillation detector isnon-reliance on extracting charged particles from the material. That is,light can be detected from the bulk of the crystal absorber with aresponse time governed by the probability of radiative decay of excitedparticles—and this can be very fast (as is the case for excitonemission).

It will be appreciated that fully inorganic perovskites may haveproperties as scintillators. As an example, nanosecond X-rayluminescence of free excitons in CsPbX3 (X=Cl, Br, I) at 77 K has beenobserved—but light yield at room temperature was <500 ph/MeV.Sub-nanosecond scintillation decay at room temperature was found inlayered hybrid metal-halide compound (C₆H₁₃NH₃)₂PbI₄—but with a lightyield of only 6000 ph/MeV. Light yields of 9000 and 14000 ph/MeV havebeen reported for layered perovskites (EDBE)PbCl₄(EDBE=2,2′-ethylenedioxy)-bis(ethylamine)) and (C₆H₅C₂H₄NH₃)₂PbBr₃.

However, even with research into OTPs as possible room temperaturescintillators with timing resolution below 10 ps—a scintillator with alight yield of at least 140000 ph/MeV and a decay time of 1 ns orshorter has remained unachievable in the currently-known art. Moreover,research into performance of OTPs as possible low temperaturescintillators has found that that decay times are worse at lowtemperature for all OTPs.

SUMMARY

Disclosed embodiments include perovskite scintillators configured to beoperated at a low temperature, detectors with perovskite scintillatorsconfigured to be operated at a low temperature, scanners with perovskitescintillators configured to be operated at a low temperature, methods ofcooling a perovskite scintillator to a low temperature, and methods ofconfiguring a perovskite scintillator to be operated at a lowtemperature.

In an illustrative embodiment, an apparatus includes a perovskitescintillator configured to be operated at a low temperature.

In another illustrative embodiment, a detector includes a source ofionizing radiation. A perovskite scintillator is configured to beirradiated by ionizing radiation at a first frequency from the source ofionizing radiation and emit photons responsive thereto at a secondfrequency that is lower than the first frequency, the perovskitescintillator being further configured to be operated at a lowtemperature. A cooling system is configured to cool the perovskitescintillator to the low temperature. A photodetector is configured todetect photons emitted by the perovskite scintillator.

In another illustrative embodiment, a scanner includes a perovskitescintillator configured to be irradiated by pairs of gamma photons at afirst frequency and emit photons responsive thereto at a secondfrequency that is lower than the first frequency, the perovskitescintillator being further configured to be operated at a lowtemperature. A cooling system is configured to cool the perovskitescintillator to the low temperature. A photodetector is configured todetect photons emitted by the perovskite scintillator.

In another illustrative embodiment, a method includes cooling aperovskite scintillator to a low temperature and irradiating the cooledperovskite scintillator with ionizing radiation.

In another illustrative embodiment, a method includes configuring aperovskite scintillator to be cooled to a low temperature.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a block diagram of an illustrative perovskite scintillatorconfigured to be operated at a low temperature.

FIGS. 1B and 1C are perspective views of details of the perovskitescintillator of FIG. 1A.

FIG. 2A is a block diagram of an illustrative detector with perovskitescintillators of FIG. 1A configured to be operated at a low temperature.

FIG. 2B illustrates details of the detector of FIG. 2A.

FIG. 3 is a side plan view in partial schematic form of anotherillustrative perovskite scintillator configured to be operated at a lowtemperature.

FIG. 4A is a block diagram of an illustrative scanner with perovskitescintillators of FIG. 1A configured to be operated at a low temperature.

FIGS. 4B-4D illustrate details of the scanner of FIG. 4A.

FIG. 5 is a side plan view in partial schematic form of anotherillustrative perovskite scintillator.

FIG. 6 is a graph that illustrates X-ray luminescence measured inillustrative perovskite scintillators at different temperatures.

FIGS. 7A and 7B are graphs that illustrate temperature dependence ofparameters of decay kinetics in illustrative perovskite scintillators.

FIG. 8A is a graph that illustrates normalized scintillation decaycurves in a perovskite scintillator and a LYSO scintillator.

FIGS. 8B and 8C are graphs of a sequence of X-ray pulses as recorded bya photon counter using a perovskite scintillator and a LYSOscintillator.

FIG. 9A is a graph of pulse height of spectra of scintillations in aperovskite scintillator.

FIG. 9B is a graph of pulse height of spectra of scintillations in a CsIscintillator.

FIG. 10 is a graph of scintillation light yield as a function oftemperature for various crystals.

FIG. 11 is a graph of photoelectric absorption of gamma rays in CsIscintillators, LYSO-Ce scintillators, and perovskite scintillators.

The use of the same symbols in different drawings typically indicatessimilar or identical items unless context dictates otherwise.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here.

Given by way of overview, various disclosed embodiments includeperovskite scintillators configured to be operated at a low temperature,detectors with perovskite scintillators configured to be operated at alow temperature, scanners with perovskite scintillators configured to beoperated at a low temperature, methods of cooling a perovskitescintillator to a low temperature, and methods of configuring aperovskite scintillator to be operated at a low temperature.

Referring to FIG. 1A, in various embodiments an illustrative perovskitescintillator 10 is configured to be operated at a low temperature. Aswill be explained below, by flying in the face of the prior art and bybeing configured to be operated at a low temperature, in variousembodiments the perovskite scintillator 10 may be able to achieve acombination of light yield and decay time that has not been achievablein the prior art.

Now that an overview has been provided, details will be provided by wayof non-limiting examples that are given by way of illustration only andnot of limitation.

Still referring to FIG. 1A, in various embodiments the perovskitescintillator 10 may include the entire material range of perovskites. Tothat end, the perovskite scintillator 10 may include any perovskite,such as, for example, without limitation a perovskite such as anorganic-inorganic trihalide perovskite or an inorganic trihalideperovskite. As is known, a perovskite has a general structure of AMX₃where:

A includes inorganic cations Ai and inorganic cations Ao. The inorganiccations Ai are independently selected from Li⁺, Na³⁰, K³⁰, Rb⁺, Cs⁺, orTl⁺, and the organic cations Ao are independently selected from ammonium(NH₄ ⁺), methyl ammonium (MA) (CH₃NH₃ ⁺), ethyl ammonium (CH₃CH₂NH₃)⁺,formamidinium (FA) (CH(NH₂)₂ ⁺), methylformamidinium (CH₃C(NH₂)₂ ⁺),guanidium (C((NH)₂)₃ ⁺), tetramethylammonium ((CH₃)₄N⁺),dimethylammonium ((CH₃)₂NH₂ ⁺), or trimethylammonium ((CH₃)₃NH⁺). A mayalso include Au, Ag, or Cu.

M is selected from Cu₂₊, Ni₂₊, Co²⁺, Fe²⁺, Mn²⁺, Cr²⁺, Pd₂₊, Cd²⁺, Ge²⁺,Su²⁺, Pb²⁺, Eu²⁺, Yb²⁺ or a combination thereof. M may also includemetal mixtures such as AgBi.

X is an anion independently selected from Br⁻, I⁻, Cl⁻, SCN⁻, CN⁻, NC⁻,OCN⁻, NCO⁻, NCS⁻, SeCN⁻, TeCN⁻, PF₆ ⁻, BF₄ ⁻ or a combination thereof.

Still referring to FIG. 1A, in various embodiments the perovskitescintillator 10 includes an organic-inorganic trihalide perovskite(“OTP”) scintillator. In some such embodiments, the OTP scintillator mayinclude an MAPbBr₃ scintillator. In other such embodiments, the OTPscintillator may include an MAPbI₃ scintillator or an MAPbCl₃scintillator. However, it will be appreciated that the perovskitescintillator 10 is not limited to an OTP scintillator or an MAPbBr₃scintillator. That is, the perovskite scintillator 10 may include theentire material range of perovskites as discussed above.

In various embodiments, timing resolution of the perovskite scintillator10 is at most 10 ps. In such embodiments, light yield of the perovskitescintillator 10 is at least 140000 ph/MeV and decay time of theperovskite scintillator 10 is at most 1 ns. Details regarding lightyield and decay time of the perovskite scintillator 10 as a function ofcooling to cryogenic temperatures will be discussed further below.

Still referring to FIG. 1A, in various embodiments a cooling system 12is configured to cool the perovskite scintillator to the lowtemperature. In some such embodiments, the cooling system 12 includes acryogenic cooling system. The cooling system 12 may be any suitablecryogenic cooling system as desired. For example, the cooling system 12may include a suitable Stirling Cryogenerator available from StirlingCryogenics, Son, Netherlands. It will be appreciated that cryogeniccooling systems are well known and, as a result, details of theirconstruction and operation need not be provided for an understanding ofdisclosed subject matter.

To that end, in various embodiments the low temperature is less than 273K. In some such embodiments, the low temperature is between about 50 and130 K, and in some such embodiments the low temperature is about 77 K.As mentioned previously, temperature-dependent behavior of theperovskite scintillator 10 is explained in detail further below.

As mentioned above, the perovskite scintillator 10 is configured to beoperated at the low temperature. As is known, perovskites arehygroscopic and exposure to moisture and oxygen can contribute todegradation of their performance. To that end and referring additionallyto FIGS. 1B and 1C, in various embodiments the perovskite scintillator10 may be encapsulated in encapsulation material 14. It will beappreciated that, as shown in FIGS. 1B and 1C, the encapsulationmaterial 14 may be disposed in any manner desired to suitablyencapsulate the perovskite scintillator 10.

In some such embodiments the encapsulation material 14 may include afilm of material such as SiO₂, Al₂O₃, SiN, other oxides, or othernitrides. In such embodiments, the film may be a thin film that isdeposited via a process such as chemical vapor deposition (“CVD”),atomic layer deposition (“ALD”), physical vapor deposition (“PVD”), orthe like.

In some other such embodiments the encapsulation material 14 may includeepoxies, polymers, ultraviolet-curable polymers, waxes or the like.

In some other such embodiments the encapsulation material 14 may includeglass. In some such embodiments, glass encapsulation or other protectivelayers may be glued to the perovskite scintillator 10. In some othersuch embodiments, the perovskite scintillator 10 may be encapsulated ina glass vial.

In some other such embodiments the encapsulation material 14 may includea two-dimensional material such as, without limitation, graphene,hexagonal boron nitride (h-BN), MoS2, or the like.

In some other such embodiments the encapsulation material 14 may includea moisture/oxygen getter material.

It will be appreciated that the perovskite scintillator 10 is suited foruse in various imaging systems, such as various detectors and scanners.Referring additionally to FIG. 2A, in various embodiments the perovskitescintillator 10 may be configured to be operated as part of a detector20. It will be appreciated that in some embodiments the detector 20 maybe an X-ray detector such as, without limitation, a medical X-raydetector, a security X-ray detector, a manufacturing inspection X-raydetector, a homeland security detector, a nuclear camera, or the like.It will be further appreciated that in some other embodiments thedetector 20 may be a gamma ray detector such as, without limitation, amedical gamma ray detector, a security gamma ray detector, anon-destructive testing radiography gamma ray or x-ray detector, apetroleum industrial gamma ray detector, or the like.

In various embodiments, the detector includes a source 22 of ionizingradiation 24. The perovskite scintillators 10 are configured to beirradiated by ionizing radiation 28 at a first frequency from the source22 of ionizing radiation 24 and emit photons 30 responsive thereto at asecond frequency that is lower than the first frequency. The perovskitescintillators 10 are further configured to be operated at the lowtemperature. The cooling system 12 is configured to cool the perovskitescintillators 10 to the low temperature. Referring additionally to FIG.2B, photodetectors 32 are configured to detect photons emitted by theperovskite scintillators 10.

Details of the detector 20 are set forth below by way of illustrationand not of limitation. It will be appreciated that the detector 20 isillustrative and may include an X-ray detector (such as withoutlimitation those described above) or a gamma ray detector (such aswithout limitation those described above).

A suitable source 22 emits ionizing the radiation 24. In someembodiments the source 22 may be an X-ray generator and the ionizingradiation 24 may be X-rays as desired for a particular application. Insome other embodiments the source 22 may be a gamma ray source and theionizing radiation 24 may be gamma rays as desired for a particularapplication. Regardless, X-ray generators and gamma ray sources are wellknown and details of their construction and operation are not necessaryfor an understanding of disclosed subject matter.

A workpiece or patient 26 is exposed to the ionizing radiation 24. Someof the ionizing radiation 26 is absorbed by the workpiece or patient 26.Ionizing radiation 28 that is not absorbed by the workpiece or patient26 is transmitted through the workpiece or patient 26. It will beappreciated that the workpiece or patient 26 is not considered to bepart of the detector 20.

The detector 20 includes the perovskite scintillators 10 that areconfigured to be operated at the low temperature. The perovskitescintillators 10 are exposed to and are irradiated by the ionizingradiation 28 that is transmitted through the workpiece or patient 26.The perovskite scintillators 10 absorb high-energy photons from theionizing radiation 28, down-convert them into the photons 30 (typicallyin the visible light frequency range), and emit the photons 30. Thus,the perovskite scintillators 10 are irradiated by the ionizing radiation28 at a first frequency and emit the photons 30 responsive thereto at asecond frequency that is lower than the first frequency. The perovskitescintillators 10 have been described above and their performance withrespect to temperature will be discussed further below. While present invarious embodiments, for purposes of clarity the encapsulation material14 (FIGS. 1B and 1C) is not shown in FIG. 2. The encapsulation material14 has been discussed above. Accordingly, further discussion of detailsof the perovskite scintillators 10 or the encapsulation material 14 isnot needed for an understanding of the detector 20.

The cooling system 12 is thermally coupled to the perovskitescintillators 10 for cooling the perovskite scintillators 10 to the lowtemperature. The cooling system 12 and the low temperature have beendiscussed above. Accordingly, further discussion of details of thecooling system 12 or the low temperature is not needed for anunderstanding of the detector 20.

The photodetectors 32 are configured to detect the photons 30. Invarious embodiments the photodetector 32 may include a photomultiplier,a photomultiplier tube, a micro-channel plate photomultiplier, a siliconphotomultiplier, an avalanche photodiode, a cadmium zinc telluridedetector, a single-photon avalanche diode, a digital siliconphotomultiplier, or the like.

Depending on the type of photodetector, performance of the photodetector32 may be affected (that is may be enhanced or may be adverselyaffected) by cooling. Because the perovskite scintillators 10 are to becooled to the low temperature, the photodetectors 32 may take advantageof this by also being cooled—although potentially to a differenttemperature. As such, in various embodiments the photodetectors 32 maybe thermally isolated from the perovskite scintillators 10—or in somecases perhaps even heated. Thus, in various embodiments thephotodetector 32 is configured to be cooled to a cooled temperature. Insome embodiments the cooled temperature may be different from the lowtemperature. In some embodiments the cooled temperature may be higherthan the low temperature.

In various embodiments, the photodetector 32 may have a coincidenceresolving time of less than 1,000 ps as desired for a particularapplication. In some such embodiments, the photodetector 32 may have acoincidence resolving time of less than 10 ps as desired for aparticular application.

In various embodiments an image processor 34 is coupled to receive andis configured to process signals that are output by the photodetectors32. The image processor 34 may be any suitable computer-based imageprocessor, image processing sub-system, or image processing system knownin the art. Image processors for detectors (like X-ray detectors andgamma ray detectors) are well known in the art, and details of theirconstruction and operation are not necessary for an understanding ofdisclosed embodiments.

Referring additionally to FIG. 3, in various embodiments severalscintillators may be combined in such a way to improve the combinedscintillator performance. In some such embodiments, the detector 20(FIG. 2) may include at least one non-perovskite scintillator 36 that isdisposed adjacent the perovskite scintillator 10 and that is configuredto be irradiated by the ionizing radiation 28 at the first frequency andto emit photons responsive thereto at the second frequency that is lowerthan the first frequency. In such embodiments, the photodetectors 32 arefurther configured to detect the photons emitted by the non-perovskitescintillator 36.

It will be appreciated that, in various embodiments, the perovskitescintillator 10 may have a fairly low density (approximately in therange of 3-4.7 g cm{circumflex over ( )}3) and a low atomic number (Znumber) compared to other scintillators—such as BGO, LYSO, LSO, and thelike. It will be further appreciated that a density of 7 g cm{circumflexover ( )}3 or more may be preferable for some applications. As is known,density or Z number (atomic number) directly correlates to the X-raystopping power of a scintillator. Thus, in various applications highstopping power or otherwise very thick crystals may be entailed—whichmay not be optimal for some applications.

To address such applications, in various embodiments the perovskitescintillator 10 may be combined with the non-perovskite (that is, highZ) scintillators 36 to take advantage of the fast response of theperovskite scintillators 10 and the high stopping power of the high Zscintillators 36 to potentially obtain advantages of both types ofscintillators. In various embodiments, such combinations ofscintillators may be in the form of stacks wherein the perovskitescintillators 10 and the high Z scintillators 36 may be stacked severaltimes on top of each other (or next to each other) or in other morecomplex configurations. In some such embodiments, the detector 20 mayinclude the perovskite scintillators 10 and the non-perovskitescintillators 36, wherein single ones of the perovskite scintillators 10are disposed adjacent single ones of the non-perovskite scintillators36. In some such embodiments, air (or other material) may be disposedbetween the alternating scintillators 10 and 36 to help reduce lightsharing.

As such, in some embodiments the non-perovskite scintillator 36 mayinclude a high atomic number (high Z) scintillator. In some suchembodiments, the high atomic number scintillator 36 may be made frombismuth germanate, lutetium oxyorthosilicate, and/or lutetium-yttriumoxyorthosilicate.

In some such embodiments, the perovskite scintillator 10 and/or thenon-perovskite scintillator 36 may be configured in a form such as aplate, a line, a square, a circle, and/or a particle. In some suchembodiments, one scintillator may be embedded in the other scintillator.In some such embodiments, the perovskite scintillator 10 and/or thenon-perovskite scintillator 36 may be defined in a topological spacesuch as zero-dimensional (“OD”), one-dimensional (“1D”), and/ortwo-dimensional (“2D”). In some such embodiments, the perovskitescintillator 10 and/or the non-perovskite scintillator 36 may be definedin a form such as nanorods, quantum dots, and/or nanocrystals.

In some such embodiments, the detector 20 may include the perovskitescintillators 10 and the non-perovskite scintillators 36. In suchembodiments, single ones of the perovskite scintillators 10 may bedisposed adjacent single ones of the non-perovskite scintillators 36. Itwill be appreciated that different types of the non-perovskitescintillators 36 (as discussed above) may be used. It will beappreciated further that the stacking configuration may be repeatedseveral times (as shown in FIG. 3).

Referring additionally to FIG. 4A, in various embodiments the perovskitescintillator 10 may be configured to be operated as part of a scanner40. In some embodiments the scanner 40 may be a tomography scanner suchas, without limitation, a positron-emission tomography (“PET”) scanner,a computed tomography (“CT”) scanner, or the like. In some otherembodiments the scanner 40 may be a scanner such as, without limitation,a scanning electron microscope, an X-ray powder diffraction system, anX-ray photoelectron spectroscope, a particle detector, or the like.

In various embodiments the scanner 40 includes the perovskitescintillator 10 configured to be irradiated by pairs of gamma photons42A and 42B at a first frequency and emit the photons 30 responsivethereto at a second frequency that is lower than the first frequency.The perovskite scintillator 10 is further configured to be operated atthe low temperature. The cooling system 10 is configured to cool theperovskite scintillator 10 to the low temperature. The photodetector 32is configured to detect the photons 30 emitted by the perovskitescintillator 10.

Details of the scanner 40 are set forth below by way of illustration andnot of limitation. It will be appreciated that the scanner 40 isillustrative and may include without limitation the scanners describedabove. For purposes of brevity, details regarding the scanner 40 will beexplained below with respect to an illustrative tomography scanner (inparticular, an illustrative PET scanner) given by way of illustrationonly and not of limitation. Details regarding the perovskitescintillators 10, the cooling system 12, and the photodetectors 32 havebeen set forth above and need not be repeated for an understanding ofdisclosed subject matter.

Referring additionally to FIGS. 4B, 4C, and 4D, in various embodiments aradioactive tracer (not shown) is injected into the body of the patient26. As is known, a tracer isotope is chemically attached to a biologicalactive molecule specific to the disease to be measured. The radioactivetracer circulates in the body of the patient 26 and accumulates ontarget cells (such as, for example, a cancerous cell). The patient 26 isinserted into the PET scanner 40.

As the radioisotope undergoes positron decay it emits a positron 44(FIGS. 4B and 4C). The positron 44 decelerates and eventually interactswith an electron 46 (FIGS. 4B and 4C). The encounter annihilates boththe positron 44 and the electron 46, thereby producing the pair ofannihilation (gamma) photons 42A and 42B that move in approximatelyopposite directions (as shown in FIGS. 4B and 4C).

The two gamma photons 42A and 42B are detected by the perovskitescintillators 10 arranged in a ring 48 in the detector 40. Due to theirhigh energy, the gamma photons 42A and 42B are difficult to be detectedby conventional detectors and therefore the perovskite scintillators 10are used to down convert the frequency of the gamma rays 42A and 42B toa frequency suitable for the photodetectors 32 (typically in the visiblefrequency range).

In various embodiments, the image processor 34 is coupled to receive andis configured to process signals that are output by the photodetectors32. In various embodiments the image processor 34 includes a coincidenceprocessing unit 50 and an image reconstruction unit 52.

The coincidence processing unit 50 is configured to localize thepositron annihilation event. In some embodiments the coincidenceprocessing unit 50 may be configured to localize the source of thepositron annihilation event along a straight line of coincidence (alsoreferred to as a line of response—or “LOR”). In embodiments in which theresolving time of the perovskite scintillators 10 is less than 500picoseconds, it may be possible to localize the positron annihilationevent to a segment of a chord (whose length is determined by the timingresolution of the perovskite scintillators 10). As the timing resolutionimproves, it will be appreciated that the signal-to-noise ratio (SNR) ofthe image will improve, thereby entailing fewer positron annihilationevents to achieve the same image quality.

In some other embodiments the coincidence processing unit 50 may beconfigured to determine an approximate position of the positronannihilation event along the line of response using measured differencein arrival times of the gamma photons 42A and 42B. Such localization isreferred to as time-of-flight positron emission tomography (“TOF-PET”).As is known, TOF-PET can help improve image quality and can help reduceimage acquisition time.

TOF-PET can be enabled in systems in which scintillator decay time is atmost around 3 ns. It will be appreciated that embodiments in which theperovskite scintillators 10 are cooled to the low temperature and decaytime is at most 1 ns lend themselves to TOF-PET applications. It will beappreciated that coincidence resolving time for photodetectors inTOF-PET applications should be on the order of hundreds of ps or, moredesirably, in the single digit ps range.

The estimated time-of-flight difference (Δt) between arrival times ofthe photons 42A and 42B at their respective perovskite scintillators 10can allow localization (with a certain probability) of the positronannihilation event on the line of response. The distance Δx to theorigin of the ring 48 of a location of the positron annihilation eventon the line of response is proportional to the time-of-flight differenceΔt according to the relationship

${\Delta x} = \frac{c\Delta t}{2}$

where c is the speed of light.

In various embodiments the image reconstruction unit 52 may beconfigured to pre-process data and reconstruct images from projections.In various embodiments, the image reconstruction unit 52 may beconfigured to reconstruct images using suitable techniques as desiredsuch as: filtered-back projection; statistical, likelihood-basediterative expectation-maximization algorithms (such as withoutlimitation the Shepp-Vardi algorithm); Bayesian methods that involve aPoisson likelihood function and an appropriate prior-probability (forexample, a smoothing prior leading to total variation regularization ora Laplacian distribution leading to A-based regularization in a waveletor other domain), such as via Grenander's Sieve estimator or via Bayespenalty methods or via Good's roughness method; or the like.

It will be appreciated that resolution of scintillators may be limitedby scintillator thickness. Referring additionally to FIG. 5, a gamma rayphoton 42 (either 42A or 42B strikes the perovskite scintillator 10 andcauses emission of the photons 30. However there can be an uncertaintyregarding where in the perovskite scintillator 10 the gamma ray photon42 hits the atom of the perovskite scintillator 10. In some cases, thisuncertainty can cause a scintillator-limited-thickness resolution givenby the relationship

Δt=H/c

whereΔt is scintillator-limited-thickness resolution;H is scintillator thickness; andc is the speed of light.

In various embodiments, this scintillator-limited-thickness resolutionis resolved by use of multiple photodetectors 32. In some suchembodiments, one photodetector 32 is disposed on one side (such as atthe top) of the perovskite scintillator 10 and another photodetector 32is disposed on another side (such as at the bottom) of the perovskitescintillator 10. Thus, with use of two photodetectors 32 the position ofthe scintillation event can be resolved and this uncertainty can bereduced.

Details regarding performance of the perovskite scintillator 10 (such asdecay time and light yield) as a function of temperature will beexplained below. In short, at lower cryogenic temperatures (such asthose toward the temperature of liquid nitrogen and below), perovskitecrystals can show scintillation properties in terms of high signaloutput and quick response time. These scintillation properties ofperovskite scintillators were measured at such temperatures using amulti-photon counting technique (and specifically—scintillation timeconstants were determined using pulsed monochromatic 14 keV X-rays fromsynchrotron radiation).

As an initial observation, the entire material range of perovskites maybe considered for use in scintillators due to independence from the bandgap. As is known, scintillators absorb high-energy radiation, farexceeding their band gap, and then emit photons. Thus, band gap tuning(that is entailed for solar cells—which are therefore limited to anarrow range of perovskites) is not entailed for scintillators. As aresult, the entire material range of perovskites can be opened forscintillation—irrespective of the band gap.

As another initial observation, perovskites contain elements with a highatomic number (high Z)—for example, Cs, Pb, I, or B. This high atomicnumber can help render perovskites as attractive scintillation materialsbecause the cross-section—and therefore scintillation—increases with Z⁴.

In an investigation of performance of perovskite scintillators at lowtemperatures, it was found that between 50 and 130 K, an MAPbBr₃ crystalexhibited a fast and intense scintillation response, with the fast(τ_(f)) and slow (τ_(s)) decay components reaching 0.1 and 1 ns,respectively. The light yield of MAPbBr₃ was estimated to be 90000±18000ph/MeV at 77 K and 116000±23000 ph/MeV at 8 K.

The scintillation light yield and decay time of MAPbBr₃ crystals wereinvestigated over the temperature range 8-295 K. It was found that atcryogenic temperatures the perovskite crystals exhibited high lightyield (>100000 ph/MeV) and sub-nanosecond decay times. This findingunderpinned the potential of OTP for detector applications that rely onfast timing of scintillation detectors at cryogenic temperatures.Synchrotron radiation was used for measurements of timingcharacteristics and a multi-photon counting technique was used formeasuring the scintillation light yield at cryogenic temperatures.

Referring additionally to FIG. 6, When excited with X-rays (50 ps pulsesof synchrotron radiation at E=14 keV), it was found that MAPbBr₃exhibits narrow, near-edge emission bands peaking at 560 nm with apronounced temperature dependence. Scintillation kinetics of MAPbBr₃crystals were studied over a wide temperature range—from 8 to 295K—using pulsed X-ray. It was found that scintillation decay curvesexhibit fast, non-exponential kinetics—which are indicative ofbimolecular recombination of charge carriers.

The main feature of the measured scintillation decay curves of theMAPbBr₃ crystal—and common for the majority of scintillationmaterials—was an increase in the decay time constant with a decrease oftemperature. It was observed that cooling the MAPbBr₃ crystal resultedin an increase of the background. Furthermore, the amplitude of thescintillation pulse initially increased with cooling but when thetemperature dropped to below 50 K it started to reduce. Inspection ofthe plots reveals that the scintillation pulse in MAPbBr₃ also undergoessignificant changes in shape at low temperatures—that is, the fractionalcontribution of the background rapidly increases and the long componentof the decay curves becomes more pronounced. These features areindicative of a slowing down in the recombination dynamics.

Referring additionally to FIGS. 7A and 7B, for a more quantitativecomparison of these properties and trends measured decay curves werefitted with a sum of exponential functions:ƒ(t)=Σ_(i)A_(i)exp(−t/τ_(i))+y_(o), where A_(i) is the amplitude, x thedecay time constant and y₀ the background. As shown in FIGS. 7A and 7B,temperature dependence of decay kinetics of MAPbBr₃ were fitted foramplitude versus temperature (FIG. 7A) and decay time versus temperature(FIG. 7B). It will be appreciated that a best fit of τ=ƒ(T) dependencieswas found with the following parameters: τ₁=1.6±0.5 ns; K₁=39±11×10⁹s⁻¹;ΔE₁=6.4±0.5 meV; τ₂=52.4±0.2 ns; K₂=18±3×10⁹s⁻¹; and ΔE₂=13.3±0.3 meV.The quality of the fit was only marginally different between two- andthree-exponential fits. Two exponentials and constant background weresufficient for an adequate representation of the measured decay curves.

An analysis of the plots reveals further details in the temperatureevolution of the luminescence kinetics of the crystal. As can be derivedfrom the decay time versus temperature dependences, the fast and slowdecay time constants in the crystal are about 0.1 and 1 ns at T>50 K.This correlates well with the results from photoluminescence decaystudies of MAPbBr₃ down to 77 K. With cooling to lower temperatures, thedecay rate of the luminescence kinetics in MAPbBr₃ exhibits steepchanges, thereby resulting in a significant increase of the decay timeconstants, so that at T=8 K, τ_(f)=2 ns and τ_(s)=50 ns. The amplitudesof the fast and slow components initially increase with cooling whilebelow 40 K they start to reduce. In particular the amplitude of the fastcomponent drops by about a factor of five. At the same time, theamplitude of the background y_(o) exhibits a steady rise with cooling,thereby becoming comparable to the amplitude of the fast component atT=8 K. This shows that at this temperature the radiative dynamics aredominated by the slow recombination processes due to trapping andrelease of charge carriers. This effect causes afterglow, which has adetrimental impact upon the temporal response of the scintillator.

Importantly at T>60 K the fast and slow scintillation componentsdominate in the radiative decay while the fractional contribution of thebackground does not exceed 1%. This implies that at the highertemperature the major fraction of the scintillation response from thecrystal is released over a nanosecond time interval (following anexcitation pulse). This is further supported through the measurements ofthe scintillation light yield discussed later. Scintillation response ofMAPbBr₃ is shown in FIG. 8A by comparing its scintillation response withthat of an LYSO-Ce scintillator. Normalized scintillation decay curveswere observed at excitation by 14 keV X-ray pulses in MAPbBr₃ (T=77 K,curve 54) in comparison with LYSO-Ce (T=292 K, curve 56). Anotherexample is given in FIGS. 8B and 8C—which display the sequence of X-raypulses from the synchrotron (FWHM=60 ps, interval Δt=2 ns) as detectedby MAPbBr₃ and LYSO-Ce. Normalized scintillation decay curves observedat excitation by 14 keV X-ray pulses in MAPbBr₃ (T=77 K—curve 58) incomparison with LYSO-Ce (T=292 K, curve 60). It is clear from FIGS. 8Aand 8B that the timing performance of the MAPbBr₃ crystal, exhibitingsub-nanosecond decay time, may be superior in comparison with LYSO-Ce.The latter exhibits a decay time constant of 33 ns and offers an exampleof one of the best results in coincidence timing resolution which relieson fast timing.

Numerous studies for the luminescence properties of MAPbX₃, X=Br and I,were conducted over a wide temperature range evidencing that free chargecarriers dominate at room temperature while excitons are stable at lowtemperature. At high-energy excitation, the thermalized electrons andholes form free excitons which in turn can interact with defects orimpurities. The narrow luminescence bands with a small Stokes shiftobserved in OTPs at low temperature are attributed to free and boundexcitons. The luminescence is very bright at low temperature butexhibits significant thermal quenching. This is due to the excitonbinding energies being tens of meV and the increase in temperaturecausing dissociation. Yet other characteristic features of excitonemission are the fast decay kinetics. The free excitons emit promptlywhile the excitations captured at defect or impurity sites recombinemore slowly through de-trapping. Consequently, the scintillationmechanism in the crystals at low temperatures is controlled by two mainprocesses that give rise to the fast and slow emission component. Thefast decay component correspond to the radiative decay of free excitonswhile the slow component of the emission is attributed to the radiativedecay of electron and holes released from the traps.

The observed temperature dependence of the luminescence decay in bothchannels can be explained in the framework of a simple quantitativemodel by considering the dynamics of radiative and non-radiativetransitions between the excited and ground states of the emissioncenter. In terms of this model the measured transition rate (the inverseof the luminescence decay constant τ) can be determined as the sum ofthe radiative (k_(r)) and non-radiative (k_(nr)) rates:

$\begin{matrix}{{\frac{1}{\tau} = {{k_{r} + k_{nr}} = {\frac{1}{\tau_{r}} + \frac{1}{\tau_{nr}}}}},} & (1)\end{matrix}$

The changes in decay time with temperature are attributed to the processof depopulation of excited state through the thermally promoted transferof excited particles over the energy barrier that leads to thenon-radiative decay. The rate associated with the non-radiative processexhibits a strong temperature dependence, thus controlling the variationof the non-radiative decay with temperature:

$\begin{matrix}{{\frac{1}{\tau_{nr}} = {K\; {\exp \left( \frac{{- \Delta}E}{kT} \right)}}},} & (2)\end{matrix}$

where K is the probability of non-radiative decay, ΔE is the activationenergy for the non-radiative transitions, and k is the Boltzmannconstant. Substituting (2) into (1) brings about the classical formula:

$\begin{matrix}{{\frac{1}{\tau} = {\frac{1}{\tau_{r}} + {K\; {\exp \left( \frac{{- \Delta}E}{kT} \right)}}}},} & (3)\end{matrix}$

Using this formula, experimental results (see FIGS. 7A and 7B) werefitted and were found to successfully describe the τ=f(T) dependenceover a range of low temperatures from 8 to 80 K. This dependenceindicates that at these temperatures the radiative decay of free andbound excitons is controlled by the thermal activation processes.However, at higher temperature there is a disparity between the modeland experimental results evidencing that the model based on theassumption of isolated emission channel is not valid anymore. It isposited that this disparity is rather an expected observation that canbe explained as following: at higher temperature, when the excitonsstart to dissociate and an electron-hole pair can escape trapping siteswithout recombination, there is a probability for particles to exchangebetween different radiative decay channels. In other words, therecombination of free and bound excitons may contribute to both emissioncomponents of luminescence decay. This effect manifests itself by anincrease of the decay time constant with heating which is also observedat photoexcitation. It is worth pointing out that OPT crystals possessexceptionally low trap densities. Consequently, radiative decay is adominant channel for the relaxation of excited states. This is one maincause for the very high luminosity at low temperatures when thermalquenching is suppressed.

Further to assess the performance of MAPbBr₃ as a scintillator, a seriesof energy spectra induced by α-particles in MAPbBr₃ were studied as afunction of temperature. FIGS. 9A and 9B show pulse height spectrummeasured at 50 K that feature a peak with Gaussian shape attributed tothe detection of 5.5 MeV α-particles emitted by an ²⁴¹Am source by anMAPbBr₃ crystal at 50 K (FIG. 9A) and a CsI crystal at 50 K (FIG. 9B).Pulse height spectra distributions of scintillations excited throughα-particle interaction from ²⁴¹Am in MAPbBr₃ at 50 K and CsI at 50 Kbefore correction for the spectral response of the photomultipliersignify scintillation response due to α-particles that are fitted byGaussians.

It will be appreciated that position of the peak center is proportionalto the amplitude of the scintillation response of the crystal so that itcan be used as a measure of scintillation light output at differenttemperatures. As shown in FIG. 10, variation of the scintillation lightoutput of the MAPbBr₃ crystal is shown with temperature (together withCsI and LYSO-Ce). A clearly measurable scintillation response can bedetected when the crystal is cooled to below 180 K. The scintillationefficiency of MAPbBr₃ increases gradually as the temperature isdecreased until a plateau is reached at around 70 K. At T>60 K theindividual scintillation event recorded by the data acquisition systemexhibits a very short, intense peak that decays within a hundrednanoseconds. This peak is caused by the overlap of many scintillationphotons arriving over a short time initial interval after excitation byan alpha particle. Hence, at these temperatures only the fast emissioncontributes to the scintillation signal. An increase of the light outputby about 20% is observed as the temperature decreases to below 30 K.This rise correlates with the rapid increase of the fractionalcontribution of the afterglow observed at very low temperatures.

It was also noted that as the temperature reduces to below 50 K, adelayed signal appears and is distributed over the entire time window ofthe 1.6 ms used to record the individual scintillation events. Thisdelayed signal is responsible for the additional emission enhancementobserved over this temperature range. It is due to the process ofradiative recombination of charge released by shallow traps withactivation energies between 10 and 90 meV as established fromthermoluminescence data.

Having demonstrated that over the 50-150 K temperature range MAPbBr₃crystals exhibit fast scintillations, and taking into consideration thetheoretical estimates, the scintillation light yield was evaluated usingas reference commercial CsI scintillators and LYSO-Ce scintillators. Asis known, in such evaluations it is preferable to use a referencescintillator with characteristics not too dissimilar to the crystalsunder study. Undoped CsI has a very high light output of 100000 ph/MeVat 77 K and exhibits strong temperature dependence, although the decaytime is relatively long (˜1 μs at 77 K). LYSO-Ce is known for its highlight yield (34000 ph/MeV) and fast decay time—both changing onlyinsignificantly with cooling. The light collection efficiency of theexperimental setup used in this study was determined predominantly bythe geometrical factors that are constant parameters. Because of lowpenetration depth, the energy of alpha particles is fully absorbed bythe thin samples, hence the scintillation light yield could be evaluatedby comparing the measured light outputs of the reference scintillatorand the perovskite crystals corrected for the difference in theemission-weighted spectral sensitivity ε_(λ).

Referring now to FIG. 10, light output of scintillators is plotted as afunction of temperature. The emission spectra of MAPbBr₃, CsI andLYSO-Ce crystals are shown in FIG. 10 and the quantum sensitivity of thephotomultiplier used for the calculation of the emission-weightedsensitivity ε_(λ) is shown in the inset of FIG. 10. Scintillation lightyield as function of temperature for the MAPbBr₃ crystal (squares) wasmeasured for excitation with 5.5 MeV alpha particles from ²⁴¹Am. Theplot also displays the comparison with measurements of commercial CsIscintillators (triangles) and LYSO-Ce scintillators (circles) with knownlight yield. The inset shows normalized emission spectra of CsI (T=77K), LYSO-Ce (T=295 K), and MAPbBr₃ (T=10 K). The dotted line is thenormalised quantum sensitivity of the photomultiplier 9124A used in themeasurements of the scintillation light yield.

Taking the light yield of CsI equal to 100000 ph/MeV at 77 K, it wasdetermined that the light yield of MAPbBr₃ is equal to 90000 ph/MeV at77 K and 116000 ph/MeV at T=8 K. On the other hand, measuring LYSO-Ce,it was found that the scintillation light yield increases to 40500ph/MeV upon cooling to T=8 K, thereby giving a light yield of MAPbBr₃equal to 110000 ph/MeV at this temperature. The estimated valuescorrelate very well despite the relatively large error ±20%, which stemsfrom the uncertainty of ε_(λ) and the determination of the centroid inthe pulse height spectra. The significance of these values can beappreciated in full when compared with the characteristics of the bestmodern scintillators (see Table 1).

TABLE 1 Properties of modern scintillation materials at roomtemperature. Data for CsI and MAPbBr₃ are shown at T = 77 K. Photoelect.Light absorption Decay Light yield/ Density, at 511 keV, Emission time,yield, decay Crystal g/cm³ cm⁻¹* peak, nm ns ph/MeV time, ns⁻¹ Csl-Tl4.5 0.09 560 1000  57000   57 Srl₂-Eu 4.6 0.07 435 1200 120000  100 Csl(77 K) 4.5 0.09 340 730/3200 100000  163 LYSO-Ce 7.1 0.25 420 33  34000 940 GGAG-Ce 6.2 0.12 540 32/156  45000  1400 BaF₂ 4.9 0.08 220/3100.8/630 1800/10000  2250 Lul₃-Ce 5.6 0.16 475;520 31/140/1000  98000 3160 LaBr₃-Ce 5.0 0.05 355;390 16  74000  4630 MAPbBr₃ 3.6 0.13 5600.1/1  90000 90000 (77 K) *-calculated using XCOM web-tool

A comparison of the MAPbBr₃ parameters with commercial scintillatorsshows that OTPs are very promising scintillation materials. Ofparticular note is the high initial photon density calculated as theratio of light yield to decay time—the most important parameter thatdetermines the timing precision of the scintillator detector. The higherdensity of photons near the peak enables a higher precision indetermining the time of interaction. A conservative evaluation showsthat this parameter is higher by a factor 20 in MAPbBr₃ compared to thebest modern scintillator LaBr₃—Ce. It should be noted that there are afew other materials with fast scintillations at cryogenic temperaturesdiscussed in literature (ZnO, PbI₂, HgI₂) but a low value of the lightyield is a major limitation. The stopping power of MAPbBr₃ that isdefined by the photoelectric fraction of the absorption coefficient isalso very competitive in comparison with other scintillators; only twomaterials exhibit a higher value. Referring additionally to FIG. 11, theenergy dependence of the photoelectric absorption of gamma rays in CsI,LYSO-Ce, and MAPbBr₃ is shown. The data were calculated using XCOMweb-tool

In summary, the decay time and light output of MAPbBr₃ crystals weremeasured down to a temperature of 8 K, using X-ray and particleexcitation. Fast and intense scintillation response—the keycharacteristics for a scintillation detector—were found. At 77 K thefast and slow components of the decay were found to be ˜0.1 ns and 1 ns,respectively. The light yield of MAPbBr₃ was estimated as 90000±18000ph/MeV at 77 K and 116000±23000 ph/MeV at 8 K. It will be appreciatedthat the advanced scintillation characteristics of OTP crystals wereattained even at moderate cooling to a temperature just below 100K—which can be achieved easily through liquid nitrogen-basedrefrigeration systems. Modern developments in cryogenics made thesetemperatures also accessible though using dry cryogenic systems whileadvances in CMOS silicon photodetectors allow reliable detection ofsingle photons at these temperatures.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

1. An apparatus comprising: a perovskite scintillator configured to beoperated at a low temperature.
 2. The apparatus of claim 1, wherein theperovskite scintillator includes an organic-inorganic trihalideperovskite (“OTP”) scintillator.
 3. (canceled)
 4. The apparatus of claim1, wherein timing resolution of the perovskite scintillator is at most10 ps.
 5. The apparatus of claim 4, wherein light yield of theperovskite scintillator is at least 140000 ph/MeV.
 6. The apparatus ofclaim 4, wherein decay time of the perovskite scintillator is at most 1ns.
 7. The apparatus of claim 1, further comprising: a cooling systemconfigured to cool the perovskite scintillator to the low temperature.8. (canceled)
 9. The apparatus of claim 1, wherein the low temperatureis less than 273 K.
 10. The apparatus of claim 9, wherein the lowtemperature is between about 50 and 130 K.
 11. The apparatus of claim10, wherein the low temperature is about 77 K.
 12. The apparatus ofclaim 1, further comprising: encapsulation material in which theperovskite scintillator is encapsulated.
 13. (canceled)
 14. Theapparatus of claim 1, wherein the perovskite scintillator is configuredto be operated as part of a detector chosen from an X-ray detector and agamma ray detector.
 15. (canceled)
 16. The apparatus of claim 1, whereinthe perovskite scintillator is configured to be operated as part of ascanner chosen from a PET scanner and a CT scanner. 17.-20. (canceled)21. A detector comprising: a source of ionizing radiation; at least oneperovskite scintillator configured to be irradiated by ionizingradiation at a first frequency from the source of ionizing radiation andemit photons responsive thereto at a second frequency that is lower thanthe first frequency, the perovskite scintillator being furtherconfigured to be operated at a low temperature; a cooling systemconfigured to cool the perovskite scintillator to the low temperature;and a photodetector configured to detect photons emitted by theperovskite scintillator.
 22. The detector of claim 21, wherein thesource of ionizing radiation includes a source chosen from an X-raysource and a gamma ray source. 23.-47. (canceled)
 48. A scannercomprising: a perovskite scintillator configured to be irradiated bypairs of gamma photons at a first frequency and emit photons responsivethereto at a second frequency that is lower than the first frequency,the perovskite scintillator being further configured to be operated at alow temperature; a cooling system configured to cool the perovskitescintillator to the low temperature; and a photodetector configured todetect photons emitted by the perovskite scintillator. 49.-60.(canceled)
 61. The scanner of claim 48, wherein the photodetector isconfigured to be cooled to a cooled temperature.
 62. The scanner ofclaim 61, wherein the cooled temperature is different from the lowtemperature.
 63. The scanner of claim 62, wherein the cooled temperatureis higher than the low temperature.
 64. The scanner of claim 48, whereinthe photodetector has a coincidence resolving time of less than 1,000ps.
 65. The scanner of claim 64, wherein the photodetector has acoincidence resolving time of less than 10 ps.
 66. The scanner of claim48, further comprising at least one non-perovskite scintillator disposedadjacent the perovskite scintillator and configured to be irradiated byionizing radiation at a first frequency from the source of ionizingradiation and emit photons responsive thereto at a second frequency thatis lower than the first frequency, the photodetector being furtherconfigured to detect photons emitted by the non-perovskite scintillator.67. The scanner of claim 66, wherein the non-perovskite scintillatorincludes a high atomic number scintillator. 68.-71. (canceled)
 72. Thescanner of claim 48, further comprising: a plurality of perovskitescintillators; and a plurality of non-perovskite scintillators, singleones of the plurality of perovskite scintillators being disposedadjacent single ones of the plurality of non-perovskite scintillators.73. The scanner of claim 48, wherein the scanner includes a tomographyscanner.
 74. The scanner of claim 73, wherein the tomography scannerincludes a positron emission tomography scanner.
 75. The scanner ofclaim 74, wherein the positron emission tomography scanner includes atime-of-flight positron emission tomography scanner. 76.-79. (canceled)80.-81. (canceled)